The present invention is directed to real time measurement of balanced or unbalanced three phase electrical parameters such as power, voltage, current and frequency. Specifically, the invention is directed to real time measurement of balanced or unbalanced three phase electrical parameters of electrical power generators using a phase-locked quadrature detector.
Polyphase and, more particularly, three-phase power generators are widely used to produce electric power for commercial and industrial use and it is highly desirable, therefore, that such generators produce AC power that is both stable and reliable. Since such generators are often subject to various types of power surges, faults, voltage and current overloads, as well as phase and frequency perturbations, it is desirable to monitor as closely as possible the output parameters of the generator. Specifically, during the operation of an electrical power generator, it is desirable for the safe operation of the system that the generator""s operating parameters be available to the operator and as a safeguard provide feedback signals to the system control in the event that there develops an unsafe condition of, for example, a mismatch between input mechanical drive power and electrical power being delivered.
Some prior art electrical power generator monitors e.g., General Electric""s Mark V Turbine Control Card DS200TCCBG1B, assume that, in a three phase system, monitored voltage and current signals have a 120 degree phase relationship between each of the voltage and current vectors. However, in the event of an unbalance in the system, the 120 degree phase relationship no longer is a valid assumption. Accordingly, any vector measurements and power calculations based on the assumed 120 degree relationship also are no longer valid.
A further presumption in other prior art generator monitors is that the frequency of power generation is maintained at a rated system frequency, e.g., 50 or 60 Hz. This assumption is generally true especially when the generator is connected a relatively large power grid whereby the generator experiences an electrical xe2x80x9cstiffnessxe2x80x9d tending to keep the frequency close to the nominal frequency. On the other hand, where an electrical power generator is not connected to a larger electrical grid, e.g., a manufacturing facility having its own generating capacity and where power machinery is turned on and off, or even in certain less developed countries where the power distribution system is not entirely reliable, 4 to 5 Hz variations in frequency from the nominal value is not uncommon. Phase unbalance and frequency variations, however, can cause undesirable effects in rotating machinery, electronic equipment and other apparatus relying on the generated electricity.
It is therefore seen to be desirable to provide an apparatus and method for determining, in real time, the key parameters of a three phase electrical generator""s output. In accordance with a preferred embodiment of the invention a pair of reference vectors in quadrature (orthogonal) is phase-locked to a selected input open delta line to line voltage vector. Preferably, the cosine reference vector is locked 45xc2x0 out of phase from the selected input vector. This allows the best projection of the input vector onto the quadrature pair providing increased accuracy in subsequent magnitude and phase calculations. The phase-locked reference vectors provide a basis for computing the magnitude and phase angle for the remaining open delta line to line voltage vectors and three line to neutral current vectors, all of which also are provided as inputs to the system of the preferred embodiment. Using the calculated magnitude and phase of selected inputs, instantaneous measurement of magnitude and phase angle for the three line to line voltages and three line to neutral currents, the generator""s real power (watts), volt-amperes reactive (VARS), apparent power (VA), power factor (PF) and, as a result of the phase locking, the precise frequency of the generator.